Systems and methods for generating a current

ABSTRACT

Systems and methods are provided for generating a current. A first current source generates a first current based on a first current selection signal, and a second current source generates a second current that is a multiple of the first current in response to selection of the first current.

BACKGROUND

Power consumption is becoming an increasing concern in the design ofintegrated circuits (ICs), particularly for very large scale integration(VLSI) chip design. Increases in power consumption are outpacing theadvantages of advances in scaling in silicon technologies, and thebenefits of reducing power supply voltages. Power management has beenrecognized as an important consideration associated with the design andoperation of VLSI (Very Large Scale Integration) chips to mitigate powerconsumption associated with VLSI chips. Consequently, approaches existto adjust the supply voltage in response to changes in current drawn bythe VLSI device.

For a variety of applications including the calibration of a powermanagement system, it is desirable to generate a precise current withamperage in the same order of magnitude as that consumed by the VLSIdevice itself. Fabrication of precise current sources on VLSI devices isvery difficult due to process variations associated with thefabrications of current sources, including those employing CMOS andBipolar-CMOS technologies. In general, process variations can includelot-to-lot variations, wafer-to-wafer variations, die-to-die variationsand within-die variations.

SUMMARY

The present invention relates generally to systems and methods forgenerating a current. One embodiment of the present invention provides asystem with a first current source having a first configuration that isselectively adjusted to achieve a first current. The system alsoincludes a second current source having a second configuration that isselected to generate a second current that is a multiple of the firstcurrent in response to a selection of the first configuration.

Another embodiment relates to an integrated circuit. The integratedcircuit comprises a first set of semiconductor devices configured toprovide a variable current source that generates a first current basedon a first selection signal. A seconds set of semiconductor devices areconfigured to provide a variable current source that generates a secondcurrent based on a second binary selection signal. The first set ofsemiconductor devices having an associated matching semiconductor devicefrom the second set of semiconductor devices that has a width that is amultiple of the width of the associated semiconductor device of thefirst set of semiconductor devices. The integrated circuit furtherincludes a control device that determines the value of the first currentand sets the second selection signal to provide the second current thatis a multiple of the first current.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an embodiment of a system for generating a current.

FIG. 2 illustrates an embodiment of a system for generating a currentemploying a first and second set of binary weighted FETs.

FIG. 3 illustrates an embodiment of an integrated circuit.

FIG. 4 illustrates an embodiment of a charge rationing system.

FIG. 5 is a flow diagram illustrating an embodiment of a methodology forgenerating a current.

DETAILED DESCRIPTION

Systems and methods are provided for generating a current (e.g., on anintegrated circuit). The systems and methods employ a first currentsource that is configured to generate a first current. A currentselector determines the value of the first current based on a voltagedrop across a precision resistor coupled between a fixed referencevoltage and the first current source. The current selector selectivelyadjusts the configuration of the first current source until a desiredfirst current is achieved. Once the desired first current is achieved, asecond current source is configured to provide a second current that isa multiple of the first current. The second current source is coupled toa supply voltage of an integrated circuit in which the first and secondcurrent sources reside. Adjustments are provided to compensate for thedifferences in the fixed reference voltage and the supply voltage. Inthis manner, fluctuations associated with the supply voltage do notaffect the selection of the first current and the second current.

FIG. 1 illustrates a system 10 for generating a current. The system 10provides for selectively adjusting a configuration of a first variablecurrent source 14 via a first selection signal ICAL_(SEL) to achieve adesired first current I_(X) based on a voltage drop across a precisionresistor R_(X). The first variable current source 14 includes thecurrent I_(X) and its associated resistance R_(CAL). A second variablecurrent source 16 is configured to provide a second current ICS that isa multiple K (e.g., K=40) of the first current I_(X) via a secondselection signal ICS_(SEL) in response to a selection of theconfiguration of the first variable current source 14. The multiple Kcan be an integer or fractional multiplier greater than one. The firstvariable current source 14 is employed to calibrate the second variablecurrent source 16. The first variable current source 14 and the secondvariable current source 16 can be formed from semiconductor devices. Thesemiconductor devices can be matched to provide the desired multiple K.

The system 10 is implemented on an integrated circuit (IC) or die 12such as a VLSI chip (e.g., a microprocessor, an application specificintegrated circuit (ASIC)) or the like. The system 10 provides for theimplementation of a large accurate current on the IC 12 in spite ofprocess variations associated with fabrication of the IC 12. The processvariations can include lot-to-lot variations, wafer-to-wafer variations,die-to-die variations as well as within-die variations. The secondvariable current source 16 can provide the second current ICS that is ofthe same order of magnitude as the current drawn by the IC 12. It is tobe appreciated that the system 10 can be implemented with discretecomponents in a non-integrated circuit.

The IC 12 includes a first terminal 20 operative to receive a fixedreference voltage V_(FIXED). A second terminal 22 is operative to becoupled to a first end of a precision resistor R_(X) off the IC 12. Asecond end of the precision resistor R_(X) is coupled to the fixedreference voltage V_(FIXED), such that the voltage drop across theprecision resistor R_(X) is provided at the first and second terminals20 and 22. The IC 12 includes a third terminal 24 coupled to a supplyvoltage of the IC 12. It is to be appreciated that the IC 12 can beformed of a plurality of portions to form a VLSI circuit, such as amicroprocessor. For example, a microprocessor circuit can be formed ofseveral portions such as one or more cores, cache memory andinput/output portion, such that the supply voltage can be operative toprovide voltage to one of the portions of the microprocessor withdifferent supply voltages and different systems 10 residing on one ormore of the portions.

A current selector 18 controls the state of the first selection signalICAL_(SEL) that controls the current selection of the first variablecurrent source 14, and the second selection signal ICS_(SEL) thatcontrols the current selection of the second variable current source 16.The current selector 18 selects a current I_(X) for the first variablecurrent source 14 associated with a desired source current of the secondvariable current source 16. The current selector 18 then measures thevoltage drop across the precision resistor R_(X), and selectivelyadjusts the current I_(X), until a voltage V_(X) at the second end ofthe precision resistor R_(X) achieves a desired voltage corresponding toa desired first current I_(X). That is, I_(X) is substantially equal toV_(FIXED)−V_(X)/R_(X). The desired voltage corresponds to a desiredsource current associated with the second variable current source 16.The second variable current source 16 provides the second current ICSthat is a multiple of the first current I_(X), such that ICS=K*I_(X) forsimilar selections of the first and second selection signals, where K isan integer or fractional multiplier greater than one. The secondvariable current source is coupled to the supply voltage V_(SUPPLY) ofthe IC 12.

For example, if it is desired to provide a current of 1 Amp from thesecond variable current source 16 and K is equal to 40, then V_(X) isadjusted until I_(X) is substantially equal to 25 mA. Once the desiredvoltage V_(X) is achieved to provide the desired current I_(X), then thesecond selection signal ICS_(SEL) is set to provides a current ICS fromthe second variable current source 16 which is substantially equal toK*I_(X). This can be accomplished by having similar configurations forboth the first and second variable current sources 14 and 16 or having aroutine that adjusts the configuration of the second.

FIG. 2 illustrates a system 30 for generating a current employing afirst and second set of binary weighted transistors. The system 30includes a first set of binary weighted Field Effect Transistors (FETs)42 configured to provide a first variable current source. The first setof binary weighted FETs 42 is operative to generate a first current I₁based on a first selection signal ICAL_(SEL). The system 30 includes asecond set of binary weighted FETs 44 configured to provide a secondvariable current source. The second set of binary weighted FETs 44 isoperative to generate a second current I₂ based on a second selectionsignal ICS_(SEL). Each FET in the first set of binary weighted FETs 42has an associated matching FET in the second set of binary weighted sets44 that has a width that is a multiple K of its associated matching FETin the first set of binary weighted FETs 42, where K is an integer orfractional multiplier greater than one. The first set of binary weightedFETs 42 and the second set of binary weighted FETs form a currentstation 40. The first set of binary weighted FETs 42 and the second setof binary weighted FETs associated with the current station 40 can beformed at a substantially same location (e.g., on an IC), so as to besubjected to similar process variation for both the first and second setof binary weighted FETs 42 and 44. Additionally, a plurality of currentstations can be distributed across an IC. It is to be appreciated thatother semiconductor devices can be employed in place of the binaryweighted FETs 42 and 44.

The first set of binary weighted FETs 42 include binary weightingassociated with the widths of the FETs. For example, a first FET has awidth of 1*W, a second FET has a width of 2*W, etc., until the final FETwhich has a width of 2^(N)*W, where N is equal to the number of FETs inthe first set of binary weighted FETs 42, where W is an arbitrary unitof width. This allows for incremental selection of a desired currentutilizing, for example, a binary N− bit word. The first FET of the firstset of binary weighted FETs sinks a current I, while the second FETsinks a current 2I, while the last FET sinks a current 2^(N)*I. Forexample, a set of eight FETs can be controlled by an 8-bit binary wordvia the first selection signal ICAL_(SEL), such that the word “00000001”would provide a current I, a word “00000010” word provide a current 2I,a word “00000011” would provide a current 3I, and a word “11111111”would provide a current 255I collectively equal to I₁.

The second set of binary weighted FETs 44 is configured to provide asecond variable current source. The second set of binary weighted FETs44 is operative to generate a second current based on a second selectionsignal IC_(SEL). Each FET in the second set of binary weighted FETs 44has a width that is a multiple K of an associated FET in the first setof binary weighted FETs. The drains of each FET in the second set ofbinary weighted FETs 44 are coupled to a supply voltage V_(SUPPLY).

The second set of binary weighted FETs 44 include binary weightingassociated with the widths of the FETs. For example, a first FET has awidth of K*1*W, a second FET has a width of K*2*W, etc., until the finalFET, which has a width of K*2^(N)*W. This allows for incrementalselection of a desired second current utilizing a similar binary N-bitword as the first set of binary weighted FETs 42. For example, the firstFET of the second set of binary weighted FETs 44 sinks a current K*I,the second FET sinks a current K*2*I, while the last FET sinks a currentK*2^(N)* I, where N is the number of FETs in the second set. Byproviding corresponding matched FETs in the first and second sets ofbinary weighted FETs 42 and 44, a first current can be derivedassociated with the first set of binary weighted FETs 42, and the secondselection signal ICS_(SEL) can be set to a similar setting as the firstselection signal ICAL_(SEL) to provide a second current from the secondset of binary weighted FETs 44 that is a multiple K of the firstcurrent.

The drains of the first set of binary weighted FETs 42 are coupled to afirst end of a precision resistor R_(X), while the second end of theprecision resistor R_(X) is coupled to a fixed reference voltageV_(FIXED). The first current associated with the first set of binaryweighted FETs 42 causes current to flow from V_(FIXED) through theprecision resistor R_(X) providing a voltage drop V_(RESDROP) across theprecision resistor R_(X). The voltage drop V_(RESDROP) is equal to thevoltage V_(FIXED) at the second end of the precision resistor R_(X)minus the voltage V_(X) at the first end of the precision resistorR_(X). The voltage V_(FIXED) is provided to a first analog-to-digitalconverter 34 and the voltage V_(X) is provided to a second ADC 35. Thefirst and second ADCs 34 and 35 convert the voltages V_(FIXED) and V_(X)from the analog to the digital domain, and provide the digital values toa difference component 37. The difference component 37 determines thevoltage drop V_(DIGDROP) in digital form associated with the analogvoltage drop V_(RESDROP) across the precision resistor R_(X).

The voltage drop V_(DIGDROP) is then provided to a comparator 36, whichcompares the digital value of the voltage drop V_(DIGDROP) with adesired voltage drop V_(DES). The desired voltage drop V_(DES) can bepredetermined or selected programmatically. The ADCs 34 and 35, thedifference component 37 and the comparator 36 form an ammeter 39. Theresults from the ammeter 37 are then provided to a current selector 32,which increments or decrements the current I₁ flowing through the firstset of binary weighted FETs 42 via the first selection signal ICAL_(SEL)based on the ammeter results. This is repeated until the desired voltageV_(X) has been achieved, and thus the selected first current has beenachieved.

Once the desired first current is achieved, the current selector 32 setsthe second selection signal ICS_(SEL) to a similar setting as the firstselection signal ICAL_(SEL), such that the same associated FETs in boththe first and second set of binary weighted FETs 42 and 44 are turned“ON” to produce a second current I₂ through the second set of binaryweighted FETs 44 that is a multiple K of the first current I₁. However,since the drain voltage (V_(X)) of first set of binary weighted FETs 42is different from the drain voltage (V_(SUPPLY)) of the second set ofbinary weighted FETs 44, the second current I₂ will not be an exactmultiple of the first current I₁. The current selector 32 employs adrain voltage offset (DVO) compensator 38 to determine the actualcurrent provided by the second set of binary weighted FETs 44. The DVOcompensator 38 can be an algorithm, or a table that employs a currentcorrection factor (V_(DSMOD)) to determine the actual current I₂associated with the second set of binary weighted FETs 44.

A set of current correction factors for different supply voltages(V_(SUPPLY)) can be determined by evaluating the I_(D)-V_(DS)characteristics (e.g., utilizing a SPICE evaluation) associated with theFETs employed in both the first set of binary weighted FETs 42, and thesecond set of binary weighted FETs 44. Table I below illustrates anexemplary set of V_(DSMOD) current correction factors associated withdifferent supply voltages (V_(SUPPLY)). The V_(DSMOD) is a channellength modulation correction factor since the drain voltage of the firstset of binary weighted FETs 42 is different than the drain voltage ofthe second set of binary weighted FETs 44. Table I illustrates a ratioof the drain current of a 0.18 um width and 0.1 um length FET, and anassociated supply voltage ranging from 0.6875 volts to 1.203125V to thesame size FET with a drain voltage of 0.95V. TABLE I V_(SUPPLY)V_(DSMOD) 0.6875V 0.9512924 0.734375V 0.9633296 0.78125V 0.97352610.828125V 0.9898123 0.875V 0.9898123 0.921875V 0.9964204 0.96875V1.002259 1.015625V 1.007468 1.0625V 1.012159 1.109375V 1.0164201.203125V 1.023925

It is to be appreciated that the V_(DSMOD) current correction factorwill vary for different channel lengths associated with the first set ofbinary weighted FETs 42 and the second set of binary weighted FETs 44.The second current associated with the second set of binary weightedFETs 44 can be determined by the following equation:ICS _(ACTUAL) =V _(DSMOD) *ICS _(SET)  EQ. 1

-   -   where ICS_(ACTUAL) is the actual current supplied by the second        set of binary weighted FETs 44, ICS_(SET) is the selected source        current, and V_(DSMOD) is the current correction factor, which        is a function of the difference between the drain voltage        (V_(SUPPLY)) of the second set of binary weighted FETs 44 and        drain voltage (V_(X)) of the first set of binary weighted FETs        42.

FIG. 3 illustrates schematic block diagram of an integrated circuit (IC)50. The IC 50 or die includes a plurality of current stations 54distributed at different locations over the IC 50. Each current station54 includes a first set of binary weighted FETs that generate a firstcurrent or calibration current and a second set of binary weighted FETsthat generate a second current or current source current. The currentstations 54 are distributed across the IC 50 or die, so that the currentsource FETs pull current in a more uniform fashion across the IC 50 ordie. The first set of binary weighted FETs of each current station 54are coupled to a current selector 52 via a first select line ICAL_(SEL),and the second set of binary weighted FETs of each current station 54are coupled to the current selector 52 via a second select lineICS_(SEL). It is to be appreciated that each of the first sets of binaryweighted FETs and the second sets of binary weighted FETs could haveassigned selection control lines for incrementing each of the setsindividually, or single selection control lines for adjusting the firstset of binary weighted FETs and/or the second sets of binary weightedFETs together.

A fixed reference voltage V_(FIXED) is coupled to the current selector52 and to a first end of a precision resistor R_(X) residing off of theIC 50. A second end of the precision resistor R_(X) is coupled to thecurrent selector 52 such that the current selector 52 can measure thefixed reference voltage V_(FIXED) and the voltage V_(X) at the secondend of the precision resistor R_(X). The voltage V_(X) is also coupledto the drains of the first set of binary weighted transistors of eachcurrent station 54. The current selector 52 adjusts the current throughthe first set of binary weighted FETs for each current station 52 basedon the measured voltage drop across the precision resistor R_(X), untila desired first current is achieved. The drains of the second set ofbinary weighted FETs for each of the current stations 52 are coupled toa current source terminal CS_(TERMINAL). In this example, the currentsource terminal CS_(TERMINAL) is coupled to a supply voltage V_(SUPPLY)through one or more device(s) 56 on the IC. However, the current sourceterminal CS_(TERMINAL) can be directly coupled to the supply voltageV_(SUPPLY) depending on the particular implementation being employed.

FIG. 4 illustrates a block diagram of a charge rationing system 70 thatemploys a variable current source. The charge rationing system 70monitors current drawn through an IC package or die and dynamicallyadjusts the power of the IC based on the current draw. A current sourceor system for generating current (not shown) as illustrated in FIGS. 1-3can be employed to calibrate the charge rationing system 70. Forexample, the current through the IC can be determined by measuring thevoltage V_(SUPPLY) at the input of the IC or die and determining thevoltage V_(PACKAGE) at one or more locations across the IC to determinea voltage drop across the package. The voltage drop across the packagecan be divided by the package resistance to determine the current drawnthrough the IC. The input voltage V_(SUPPLY) can be dynamically adjustedvia a voltage supply control V_(CNTRL) based on the current measurementsmade during operation of the IC to mitigate power consumption and tofacilitate operational efficiency.

The system for generating a current (not shown) provides a known fixedcurrent that can be employed to calibrate the charge rationing system70. A charge rationing controller 84 can be employed to set the currentof both the first variable current source and the second variablecurrent source utilizing a similar technique as described with respectto the current selector 32 of FIG. 2. During a first stage, “A” inputsof a first multiplexer 72 and a second multiplexer 78 are provided tooutputs of the respective multiplexers based on a state of multiplexerselect lines (SEL). A voltage V_(X) at a node that couples a first endof a precision resistor to a first variable current source is providedat the output of the first multiplexer 72, and a fixed reference voltageV_(FIXED) that is coupled to a second end of the precision resistor isprovided at the output of the second multiplexer 78 during calibration.

The output of the first multiplexer 72 is coupled to a first voltagecontrolled oscillator (VCO0) 74. The first voltage controlled oscillator74 provides a first oscillating signal having a frequency associatedwith the voltage level of the voltage V_(X). A first counter (COUNTER0)76 increments over a predetermined period of time to a count thatcorresponds to the frequency of the first oscillating signal. In thismanner, the first voltage controlled oscillator 74 and the first counter76 function as an A/D converter to provide a digital count value thatcorresponds to the voltage V_(X). The count value (CNT0) of the firstcounter is then provided to the charge rationing controller 84.

The output of the second multiplexer 78 is coupled to a second voltagecontrolled oscillator (VCO1) 80. The second voltage controlledoscillator 80 provides a second oscillating signal having a frequencyassociated with the voltage level of the voltage V_(FIXED). A secondcounter (COUNTER1) 82 increments over the same predetermined period oftime as the first counter 76 to a count that corresponds to thefrequency of the second oscillating signal. In this manner, the secondvoltage controlled oscillator 80 and the second counter 82 function asan A/D converter to provide a digital count value that corresponds tothe voltage V_(FIXED). The count value (CNT1) of the second counter 82is then provided to the charge rationing controller 84.

The charge rationing controller 84 determines the voltage differencebetween the V_(X) count value and the V_(FIXED) count value. A currentassociated with the first current source can then be determined bydividing the voltage difference by the value of the precision resistor.The charge rationing controller 84 determines if a desired first currenthas been achieved. The charge rationing controller 84 can adjust thefirst variable current source employing a selection signal ICAL_(SEL)and repeat the measurement process, until the desired first variablecurrent from the first variable current source is achieved. Once thedesired first variable current from the first variable current source isachieved, the charge rationing controller 84 sets the second variablecurrent source via a select line IC_(SEL) to provide a second currentthat is a multiple of the first current. The charge rationing controller84 then employs a voltage compensator (not shown) to compensate for thedifferences in the voltages supplying power to the first and secondvariable current sources to determine the actual second current. Theactual second current can be employed to calibrate the charge rationingsystem 84.

During a second stage, “B” inputs of a first multiplexer 72 and a secondmultiplexer 78 are provided to outputs of the respective multiplexersbased on a state of multiplexer select lines (SEL). The voltage from theinput voltage V_(SUPPLY) is provided to the output of the firstmultiplexer 72, while the voltage V_(PACKAGE) is provided at the outputof the second multiplexer 78. A normal voltage drop across the IC can bemeasured with the second current in an “OFF” state. The first voltagecontrolled oscillator 74 provides a first oscillating signal having afrequency associated with the voltage level of the voltage V_(SUPPLY).The first counter 76 increments over a predetermined period of time to acount that corresponds to the frequency of the first oscillating signal.Similarly, the second voltage controlled oscillator 80 provides a secondoscillating signal having a frequency associated with the voltage levelof the voltage V_(PACKAGE). The second counter 82 increments over thepredetermined period of time to a count that corresponds to thefrequency of the second oscillating signal. A difference value D1 isdetermined associated with the voltage drop across the package.

The second current source is turned to the “ON” state. Again, the firstvoltage controlled oscillator 74 provides a first oscillating signalhaving a frequency associated with the voltage level of the voltageV_(SUPPLY). The first counter 76 increments over a predetermined periodof time to a count that corresponds to the frequency of the firstoscillating signal. Similarly, the second voltage controlled oscillator80 provides a second oscillating signal having a frequency associatedwith the voltage level of the voltage V_(PACKAGE). The second counter 82increments over the predetermined period of time to a count thatcorresponds to the frequency of the second oscillating signal. Adifference value D2 is determined associated with the voltage dropacross the package with the second current source “ON”. The packageresistance for the IC can be determined by evaluating the followingexpression:R _(PKG)=(D2−D1)/ICS  EQ. 2

This value can be stored for use during normal operation. During normaloperation the IC power can be determined by evaluating the expression:P=V _(DIE) *V _(PKGDROP) /R _(PKG)  EQ. 3where V_(PKGDROP)=V_(PACKAGE)−V_(DIE).

In view of the foregoing structural and functional features describedabove, a methodology will be better appreciated with reference to FIG.5. While, for purposes of simplicity of explanation, the methodologiesof FIG. 5 are shown and described as being implemented serially, it isto be understood and appreciated that the present invention is notlimited to the illustrated order, as some aspects could, in accordancewith the present invention, occur in different orders and/orconcurrently with other aspects from that shown and described. Moreover,not all illustrated features may be required to implement a methodology.It is to be further understood that the following methodology can beimplemented in hardware, software (e.g., computer executableinstructions), or any combination thereof.

FIG. 5 illustrates a methodology for generating a current. Themethodology begins at 100 where a first variable current source is setto generate a first current. The first variable current source iscoupled to a fixed reference voltage through a precision resistor. Thefirst variable current source can be one or more sets of binary weightedFETs that provide incremental current values based on the value of abinary N-bit selection signal, where N is the number of FETs in a set.At 110, the voltage drop across the precision resistor is measured todetermine a voltage associated with the first current. At 120, theactual first current is determined based on the measured voltage drop.For example, the voltage drop can be measured employing an ADC todetermine a digital value that corresponds to the voltage drop. Thedigital value of the voltage drop and the value of the precisionresistor can be employed to determine an actual current sourced by thefirst variable current source. The methodology then proceeds to 130.

At 130, it is determined whether or not the desired first currentassociated with the first variable current source has been achieved. Ifthe first current associated with the first variable current source hasnot been achieved (NO), the methodology returns to 100 to adjust thesetting of the first current source until the desired first current hasbeen achieved. If the first current associated with the first variablecurrent source has been achieved (YES), the methodology proceeds to 140.

At 140, a second variable current source is set to generate a secondcurrent that is a multiple of the first current. The second variablecurrent source can be one or more sets of binary weighted FETs thatprovide incremental current values based on the value of a binary N-bitselection signal, where N is the number of FETs in a set. Furthermore,the one or more binary weighted FETs associated with the second variablecurrent source can have matching FETs associated with FETs in the firstcurrent source that have weightings that are a multiple K of theassociated FETs in the first variable current source. For example, theFETs in the second variable current sources can have widths that are Ktimes the widths of the associated first FETs, such that a secondcurrent is provided that is K times the first current for a similarselection of FETs. The second variable current source is poweredemploying a supply voltage (e.g., of an integrated circuit). Themethodology then proceeds to 150.

At 150, compensation for voltage differences associated with voltagesfor powering the first and second current variable current sources isdetermined. For example, if the first and second variable currentsources are binary weighted FETs, then the voltages differencecorrespond to the different drain voltages associated with the first andsecond set of binary weighted FETs. The compensation can be a set ofcurrent correction factors matched with different values of supplyvoltages. At 160, the actual second current is determined by applyingone or more compensation factors to the selected second current.

What have been described above are examples of the present invention. Itis, of course, not possible to describe every conceivable combination ofcomponents or methodologies for purposes of describing the presentinvention, but one of ordinary skill in the art will recognize that manyfurther combinations and permutations of the present invention arepossible. Accordingly, the present invention is intended to embrace allsuch alterations, modifications and variations that fall within thespirit and scope of the appended claims.

1. A system for generating a current, comprising: a first current sourcehaving a first configuration that is selectively adjusted to achieve afirst current; and a second current source having a second configurationthat is selected to generate a second current that is a multiple of thefirst current in response to a selection of the first configuration. 2.The system of claim 1, further comprising a precision resistor coupledbetween the first current source and a fixed reference voltage.
 3. Thesystem of claim 2, further comprising a current selector that determinesthe first current based on the voltage drop across the precisionresistor and selectively adjusts the first current selection signal to adesired first current.
 4. The system of claim 3, the current selectorprovides a second current selection signal to select the second currentupon achieving the desired first current.
 5. The system of claim 1, thefirst current source comprising a first set of Field Effect Transistors(FETs) coupled to provide the first current and the second currentsource comprising a second set FETs coupled to provide the secondcurrent.
 6. The system of claim 5, wherein each FET from the first setof FETs is associated with a matching FET from the second set of FETshaving a width that is K times the width of the matching FET from thefirst set of FETs, where K is an integer greater than one.
 7. The systemof claim 6, the first set of FETs and the second set of FETs define acurrent station, and further comprising a plurality of additional firstset of FETs and additional second set of FETs defining a plurality ofcurrent stations.
 8. The system of claim 7, the plurality of currentstations being distributed at different locations across an integratedcircuit, such that the second current is drawn more uniformly across theintegrated circuit.
 9. The system of claim 5, the first set of FETshaving a common drain coupled to a first voltage and the second set ofFETs having a drain coupled to a second voltage, and further comprisinga drain voltage offset compensator that compensates for the differencein drain voltage between the first and second sets of FETs.
 10. Anintegrated circuit comprising the system of claim
 1. 11. The integratedcircuit of claim 10, the first current source is associated with areference voltage and the second current source is associated with asupply voltage of the integrated circuit.
 12. The system of claim 1,further comprising a charge rationing system that employs the secondcurrent to calibrate the charge rationing system by measuring thedifference with the second current source in an “OFF” state and thesecond current source in an “ON” state.
 13. An integrated circuitcomprising: a first set of semiconductor devices configured to provide avariable current source that generates a first current based on a firstbinary selection signal; a second set of semiconductor devicesconfigured to provide a variable current source that generates a secondcurrent based on a second binary selection signal, each of thesemiconductor devices of the first set of semiconductor devices havingan associated matching semiconductor device from the second set ofsemiconductor devices that has a width that is a multiple of the widthof the associated matching semiconductor device from the first set ofsemiconductor devices; and a control device that determines the value ofthe first current and sets the second binary selection signal to providethe second current that is a multiple of the first current.
 14. Theintegrated circuit of claim 13, the first set of semiconductor devicesare coupled to a fixed reference voltage through a precision resistorand the second set of semiconductor devices are coupled to a supplyvoltage of the integrated circuit.
 15. The integrated circuit of claim13, the control device determines the value of the first current bymeasuring the voltage across the precision resistor and determines thecurrent flowing through the precision resistor based on the measuredvoltage and resistance of the precision resistor.
 16. The integratedcircuit of claim 13, the first set of semiconductor devices comprising afirst set of Field Effect Transistors (FETs) coupled to provide thefirst current and the second the first set of semiconductor devicescomprising a second set of FETs coupled to provide the second current.17. The integrated circuit of claim 16, further comprising a drainvoltage offset compensator having at least one current correction factorto compensate for different drain voltages associated with the first setof FETs and the second set of FETs, the controller employs the at leastone current correction factor to determine an actual second currentassociated with the second variable current source.
 18. The integratedcircuit of claim 17, the drain voltage offset compensator comprising adrain voltage compensation table.
 19. The integrated circuit of claim18, further comprising a plurality of current stations, each of thecurrent stations having a first set of FETs and a second set of FETs,the plurality of current stations being distributed at differentlocations across the integrated circuit.
 20. A system for providing avariable current on a Very Large Scale Integrated (VLSI) circuitcomprising: means for generating a first current; means for selectivelyadjusting the first current to achieve a desired first current; meansfor selecting a desired second current based on the achieved desiredfirst current; means for generating a second current that is a multipleof the first current; means for compensating for differences in voltagesassociated with powering the means for generating a first current andthe means for generating a second current.
 21. The system of claim 20,the means for selectively adjusting the first current to achieve adesired first current comprising means for measuring a voltage across aprecision resistor coupled to a fixed reference voltage and the meansfor generating a first current, and means for evaluating the firstcurrent based on the measured voltage and resistance of the precisionresistor.
 22. A current generation method for an integrated circuit,comprising: generating a first current through a precision resistor;determining the value of the first current by measuring the voltageacross the precision resistor and evaluating the value of the firstcurrent based on the measured voltage and resistance of the precisionresistor; selectively adjusting the first current and determining thevalue of the first current until a desired first current is achieved;and generating a second current that has a value that is a multiple ofthe achieved first current.
 23. The method of claim 22, the generating afirst current comprising selecting at least one of a first set of FieldEffect Transistors (FETs), and the generating a second currentcomprising selecting at least one matching FET from a second set ofFETs.
 24. The method of claim 23, further comprising determining anactual second current by compensating for a difference in drain voltageassociated with the first set of FETs and the second set of FETs.
 25. Acurrent source comprising: a plurality of current stations distributedover different locations on an integrated circuit, each of the pluralityof current stations having a first set of semiconductor devices and asecond set of semiconductor devices, each of the semiconductor devicesfrom the first set of semiconductor devices having a matchingsemiconductor device from the second set of semiconductor devices thathas a width that is a multiple K of the width of the correspondingmatching semiconductor device of the first set of semiconductor devices;a first select signal associated with selecting a first current to besourced by the first set of semiconductor devices; a second selectsignal associated with selecting a second current to be sourced by thesecond set of semiconductor devices; and a current selector thatcontrols the state of the first select signal and the second selectsignal, and selects the first current sourced by the first set ofsemiconductor devices and the second current source by the second set ofsemiconductor devices.
 26. The current source of claim 25, the first setof semiconductor devices comprising a first set of Field EffectTransistors (FETs) coupled to provide the first current and the secondset of semiconductor devices comprising a second set of FETs coupled toprovide the second current.
 27. The current source of claim 26, furthercomprising a precision resistor coupled between a fixed referencevoltage and drains of the first set of FETs of each current station, andcoupling a supply voltage to drains of the second set of FETs of eachcurrent station.